Other Structural Modifications

The lignans are a large group of plant phenolics, biosynthesized from the union of two phenylpropane molecules e.g., both matairesinol (Centaurea species, family Asteraceae) and podophyllotoxin Podophyllum peltatum, family Berberidaceae) are formed from the phenylpropane coniferyl alcohol. Lignans are essentially cinnamoyl alcohol dimers, though further cyclization and other structural modifications result in various structural types, e.g. dibenzylbutyrolactone and epoxy lignan. 

Granting that a tricyclic ring system takes precedence over a tetracyclic ring in simplicity, the other structural modifications, namely introduction of oxygen functions and iinsaturation, are high order deviations from the simple hydrocarbon. However, we are concerned only with compounds containing the steroid skeleton. 

The liver, and also bacteria in the small and large intestine, can cause other structural modifications to bile acids as they undergo their entero-hepatic cycle. The formation of sulfate esters, already mentioned with respect to lithocholate in Section 4.2.1, is carried out primarily in the liver in man by a sulfotransferase (Lll). Other bile acids can also be sulfoconjugated to a small extent, mainly at the 3a-hydroxyl position. Bacteria, which have been isolated anaerobically from human feces, are known to possess bile acid sulfatase activity, which removes the 3a-sul te group of chenodeoxycholic and cholic acids (H24). The action of this bacterial enzyme probably explains why only trace amounts of sul ted bile acids, which are poorly absorbed in the intestine, are detected in the feces (12). Another type of bile acid conjugate, which has been identified in the urine of healthy subjects and patients with hepatobiliary disease, is the glucuronide (A7, S41). Both the liver and extrahepatic tissues, such as the kidney and small intestinal mucosa, are capable of glucuronidation of bile acids in man (M14). 

Thus, although these water soluble naphthalocyanines have favorable photophysical properties, their tendency still to aggregate in water and to photobleach readily precludes further development for PDT. No doubt other structural modifications can be incorporated to preclude these problems. 

When 4-methylmercapto is substituted for 4-chloro, and 2-propionic acid for acetic acid, compounds less active than indomethacin are produced . Other structural modifications have been discussed by Shen . 

The cyclic 11,12-carbonate of erythromycin represents a previously recognized method for stabilization of erythromycin by maintaining an equilibrium between the 6-hydroxy-9-keto and 6,9-hemiketal forms [44]. A new direction within this approach was recently reported with a series of cyclic 11,12-carbamate derivatives of erythromycin and clarithromycin (see Fig. 5), prepared by a general sequence of 10,11-dehydration, 12-0-carbamoylation, and intramolecular cyclization [45, 46]. Other structural modifications within this part of the erythromycin molecule include 10,11-anhydroerythromycin, previously synthesized from the cyclic 11,12-carbonate [47],... 

Fig, 6 Structurally characterized examples of isomers with different X-X bond lengths and other structural modifications... 

Interested by the effect of other structural modifications on the thermal behavior, and the possibility to combine several metal centers within the same compound, Swager and co-workers pursued this research, and reported series of homo-dinuclear, hetero-dinuclear 1,3,5-triketonate (79), and 1,3,5,7-tetraketonate Schiff-base complexes (80). " Providing a minimum of at least six lateral chains and short diamine spacers ((79) M, M = Cu, Ni, Mn, Pd, Co X = C2H4, CsHg), complexes (79) displayed Colh mesophases, being as such the first heterodinuclear metallomesogens (Table 48). The mesophase existed over broader temperature ranges than those of the related... 

IR technique has been used for qualitative analysis via comparison evaluation of the spectra of the polymer samples obtained from modification with the original one. Chitin/chitosan samples obtained from de-acetylation or acetylation process may be used for qualitative evaluation of DA and other structural modifications (Sannan et al. 1976, Domard and Rinaudo 1983, Focher et al. 1990, Kim et al. 1996, Knaul et al. 1998). The samples may be obtained from fragmentation or depolymerization process (Focher et al. 1992a, Kasaai et al. 1999).The effects of modifications on the chemical structures and the DA can be found elsewhere (Sannan et al. 1976, Domard and Rinaudo 1983, Focher et al. 1990, 1992a,b, Kim et al. 1996, Saito et al. 1997, Knaul et al. 1998, Kasaai et al. 1999). 

Both frequency and amplitude are kept constant. The only variable parameter is the temperature [dynamic mechanical thermal analysis (DMTA) test]. This type of test is used to examine the temperature dependence of materials in terms of phase changes or other structural modification. 

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